Coated metallic sheet material and method of making the same



3,054,732 COATED METALLIC SHEET MATERIAL AND METHOD OF MAKING THE SAME James M. McQuade, Pittsfield, Mass, assignor to General Electric Company, a corporation of New York No Drawing. Filed Mar. 5, 1959, Ser. No. 797,344 3 Claims. (ill. 20437) The present invention relates to coated metallic sheet materials and to the method of producing the same. More particularly, the invention concerns electrically insulating coatings for magnetic sheet material and to an improved process for applying such coatings.

The forms of magnetic sheet material with which the invention is principally concerned include strip material such as used in wound transformer cores, and cut or punched laminations forming stacked transformer cores and other electrical apparatus. A particular magnetic sheet material which may be effectively coated in accordance with the invention is magnetic silicon steel which in 'a conventional composition contains about 3% silicon, 12% manganese, .03% carbon, .01% sulfur, and 01% phosphorus, it being understood, however, that the composition of the magnetic silicon steel used in practicing the present invention need not correspond to the specified values.

In the process of treating the magnetic sheet materials to adapt them for use in transformers or other electrical devices, the sheet material is generally wound in the form of a roll or cut and arranged into a plurality of stacked sheets, and placed in these forms in an annealing furnace for the purpose of developing the magnetic properties of the sheet material. During the heat treatment in the furnace with the sheet material either in Wound or stacked form, the adjacent surfaces of the magnetic sheet material are in contact with each other over comparatively large areas with the result that at the elevated temperatures employed for developing the magnetic qualities of the material, the adjacent laminations or turns of the material tend to stick together unless some means is provided for separating the surfaces during the heat treatment.

Further, it is desirable for an insulating barrier to be provided between the adjacent laminated sheets of the magnetic material in order to reduce the eddy current losses in the core formed by the sheet material in its use in a transformer, motor or the like.

Coating compositions used in the past to provide interlaminar insulation for the above purposes have been subject to various drawbacks, including the tendency to rub oil on contact, the lack of uniformity in thickness with resultant poor space factor, the introduction of contaminants such as carbon into the metal being coated, or the lack of adequate insulation qualities after the high temperature annealing treatment.

It is an object of the present invention to provide an improved coating of the above type on magnetic silicon steel sheet material and an improved process for producing the same.

It is a particular object of the invention to provide for the application of a refractory insulating coating on magnetic silicon steel sheet laminations by a process which is readily controlled, easily and economically carried out, and adapted for large scale production methods.

In attaining the above objects and advantages, and others which will become apparent from the description herein, the invention in its broad aspects provides a method of producing an insulating coating on the surface of a magnetic silicon steel body which comprises the deposition of an adherent metal hydroxide coating on the surface of the silicon steel body by electrolyzing an aqueous solution of a water soluble metal salt from which insoluble 3,054,732 Patented Sept. 18, 1962 hydroxides are deposited on the silicon steel body forming the cathode. The silicon steel body thus coated with a thin adherent metal hydroxide coating is then subjected to elevated temperatures during which the hydroxide deposit is converted to a refractory insulating coating.

Particularly preferred compounds for forming the insulating coatings are water soluble calcium and magnesium salts, e.g., calcium nitrate or magnesium nitrate, from 0 which the insoluble calcium hydroxide or magnesium hydroxide may be deposited in accordance with the invention. The conventional high temperature annealing treatment of silicon steel strip on which the hydroxide coatings are applied as described results in the formation of a silicate film at the interface of the hydroxide deposit and the metal surface due to the reaction between the hydroxide and the silicon in the iron. This silicate film, which is a glass-like, highly insulating layer, serves in the final assembly of the silicon steel laminations as interlaminar insulation to reduce eddy current losses. During the annealing treatment, the remaining hydroxide deposit decomposes to yield the corresponding refractory oxide, e.g., calcium oxide, which separates the strips during pack annealing and thus prevents sticking of the strips to one another.

The deposition of calcium hydroxide by the electrolysis of a calcium nitrate solution has proved exceptionally satisfactory in producing thin extremely adherent coatings of uniform thickness on the metal cathode surfaces. Other soluble calcium salts, however, such as calcium chloride and other calcium halides may be used for practicing the invention if desired. Magnesium nitrate is also particularly suitable for use in practicing the invention, but other soluble magnesium salts which may be used include, but are not restricted to, magnesium acetate, magnesium sulfate and magnesium halides.

Soluble salts of other refractory producing metals may be employed, notably those of aluminum and manganese, as, for example, aluminum nitrate, manganese sulfate and manganese nitrate.

Mixtures of the various salts mentioned above may also be used in preparing the solution to be electrolyzed;

The concentration of the soluble salt in the aqueous solution is not critical and may vary widely from a very small value, say, 0.1 molar, up to a saturated solution. A satisfactory concentration for most practical purposes has been found to be about 0.8 molar. This Value, of course, will vary depending on the particular salt, the solution temperature, and other factors.

The following examples are set forth as illustrative of the process of the present invention, it being understood that the scope of the invention is not intended to be limited thereby: Example I An 0.8 molar solution of Ca(NO was electrolyzed in an electrolytic bath, using a silicon-iron sheet 5% inches wide x 6 inches long as the cathode and platinum as the anode. In the experiment, 8 volts were applied across the terminals with a current density of amps./ft. the electrolyzing period being for 30-60 seconds. In this and the other examples described herein, the spacing of the electrodes was 3 inches. Using the above procedure, adherent coatings of Ca(OH) ranging in thickness from .05 to .20 mil were obtained on a number of samples of silicon-iron sheets.

The thus coated sheets were annealed in hydrogen at 1175 C. for 8 hours. Franklin insulation values for these samples averaged 0.2 ampere.

The Franklin insulation values mentioned herein were determined by the standard Franklin test for determining the value of insulation of coatings of the described type, and in this test readings of 1 ampere represent no surface insulation and 0 ampere represent perfect insulation. In

general, Franklin insulation values below 0.4 ampere are considered entirely satisfactory for the purposes of the invention.

Example 11 A 1.2 molar solution of Mg(NO was electrolyzed, using stainless steel of 18% chromium, 8% nickel, remainder iron as the anode and a silicon-iron sheet as the cathode, the applied voltage being 3.8 volts with a current density of 75 amperes per square foot. The solution was electrolyzed for 50 seconds, and a coating of Mg(OH) of 0.4 mil thickness was deposited on the silicon steel sheet. The Franklin insulation values of several samples after annealing for 8 hours at 1175 C. in

hydrogen average 0.2 ampere.

Example III Example IV Silicon-iron sheet material was successfully coated with a mixture of Ca(OH) and Mg(OH) by electrolyzing a number of solutions containing various amounts of Ca(NO and Mg(NO Best results were obtained where the weight ratio of Mg(NO to Ca(NO did not exceed 1 to 10. An aqueous solution containing a l to 20 weight ratio of Mg(NO to Ca(NO was electrolyzed at a current density of 70 amperes per square foot, and a coating mixture of Ca(OH) and Mg(OH) was depositedin a thickness of about 0.3 mil per side after a period of 60 seconds. Franklin insulation values after an ll75 C. anneal in hydrogen averaged 0.3

ampere.

Example V Using a platinum anode and a silicon-iron Sheet as cathode, a saturated solution of MnSO; was electrolyzed for 45 seconds, thereby obtaining a 0.15 mil deposit of white Mn(OH) on the cathode. A current density of 70 amps/ft. was used. Upon drying the coating, it turned black, apparently forming Mn The amount of coating material deposited in a given period depends on such factors as the current density, the concentration of the solution, and the size and spacing of the electrodes. Accordingly, suitable variation of these factors provides a means for readily controlling the coating thickness to obtain the desired results.

The mechanism of the deposition by the described electrolyzing process apparently involves the discharge of hydrogen ions at the cathode, which results in an increased hydroxyl ion concentration at the surface of the cathode. In the case where soluble salts of calcium are present in the solution, calcium hydroxide will precipitate on the cathode forming an adherent coating. With the use of an inert anode, such as platinum, in combination with an aqueous solution of Ca(NO the overall reaction may be written as:

The oxygen is evolved in this reaction at the anode and results from the oxidation of hydroxyl ions at the anode. The hydrogen is evolved at the cathode.

In view of the production of nitric acid in this reaction which tends after a period of time to make the solution unduly acid, solid Ca(OH) powder may be added if desired to the solution to neutralize the nitric acid. Such addition will serve not only to keep the pH 4 constant but also to keep the concentration of the Ca(NO at its original value.

While inert anodes are preferred for use in the present process, such as platinum, anodes of other materials may be used provided they do not contaminate or otherwise unfavorably afiect the coating deposited on the cathode. Aluminum, for example, could be satisfactorily employed as anode material in coating Silicon steel for electrical purposes.

In the case where magnesium nitrate is used in the electrolyte solution the chemical reaction is apparently difierent from that of the calcium salt. The reaction in this case may be written as:

Here the hydrogen ions are depleted by reaction with the nitrate ions, thus making the solution basic and resulting in the deposition of Mg(OH) on the cathode. It is to be noted that the mechanism of the present electrolyzing process is not simply the migration (electrophoresis) of charged particles to the cathode for deposition of a coating thereon. The subject process involves not only the electrolyzing of the soluble salt but also a subsequent chemical reaction, as indicated above, to provide the necessary conditions for forming and precipitating the insoluble hydroxide and depositing it on the surface of the metal cathode sheet. It appears that the excellent adherence as well as the uniform thickness of the deposited coating is very likely due to the very small particle size of the hydroxy compound as it precipitates out of the solution.

There is thus provided by the invention a process which enables the production of tightly adherent insulating coatings on magnetic sheet material in which the coating thickness is easily controlled, which is adapted for continuous and large scale production, and which overcomes the disadvantages of prior art coatings as previously mentioned.

While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the scope of the invention. Therefore, the appended claims are intended to cover all such equivalent variations as come within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of producing electrically insulated magnetic silicon steel sheet material which comprises electrolyzing an aqueous solution consisting essentially of at least one com-pound selected from the group consisting of water soluble salts of calcium, magnesium, manganese, and aluminum, with the silicon steel sheet material be ing arranged as the cathode in said solution, for forming and depositing an adherent coating of the hydroxide of the cation of said compound on said silicon steel sheet material, removing the thus coated silicon steel sheet material-from said solution, and heating said coated sheet material at elevated temperature to react said hydroxide with the silicon in said sheet material for forming a refractory insulating coating comprising a silicate compound on the surfaceof said sheetmaterial;

2. The method of producing electrically insulated magnetic silicon steel sheet material which comprises electrolyzing an aqueous solution consisting essentially of at least one compound selected from the group consisting of Water soluble salts of calcium, magnesium, manganese, and aluminum, with the silicon steel sheet material being arranged as the cathode in said solution, for forming and depositing an adherent coating of the hydroxide of the cation of said compound on said silicon steel sheet material, removing the thus coated silicon steel sheet material from said solution, and heating said coated sheet material at about ll75 C. to react said hydroxide with 5 the silicon in said sheet material for forming a refractory insulating coating comprising a silicate compound on the surface of said sheet material.

3. Electrically insulated magnetic sheet material comprising a magnetic silicon steel sheet having thereon a thin, tightly adherent coating of the reaction product of the silicon in said sheet material and a water insoluble metal hydroxide deposited on said sheet material by electrolysis of an aqueous solution of at least one compound selected from the group consisting of water soluble salts of calcium, magnesium, manganese, and aluminum, said reaction product comprising a permanent, electrically insulating, refractory layer composed of a silicate com- 6% pound firmly bonded to the surface of the silicon steel sheet material.

References Cited in the file of this patent UNITED STATES PATENTS 571,531 Langhans Nov. 17, 1896 2,394,047 Elsey et a1. Feb. 5, 1946 2,534,234 Cox Dec. 19, 1950 FOREIGN PATENTS 264,534 Germany Sept. 13, 1913 914,337 Germany July 1, 1954 

1. THE METHOD OF PRODUCING ELECTRICALLY INSULATED MAGNETIC SILICON STEELL SHEET MATERIAL WHICH COMPRISES ELECTROLYZING AND AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF WATER SOLUBLE SALTS OF CALCIUM, MAGNESIUM, MANGANESE, AND ALUMINUM, WITH THE SILICON STEEL SHEET MATERIAL BEING ARRANGED AS THE CATHODE IN SAID SOLUTION, FOR FORMING AND DEPOSITING AN ADHERENT COATING OF THE HYDROXIDE OF THE CATION OF SAID COMPOUND ON SAID SILICON STEEL SHEET MATERIAL, REMOVING THE THUS COATED SILICON STEEL SHEET MATERIAL FROM SAID SOLUTION, AND HEATING SAID COATED SHEET MATERIAL AT ELEVATED TEMPERATURE TO REACT SAID HYDROXIDE WITH THE SILICON IN SAID SHEET MATERIAL FOR FORMING A REFRACTROY INSULATING COATING COMPRISING A SILICATE COMPOUND ON THE SURFACE OF SAID SHEET MATERIAL. 